You are a world-famous physicist-lawyer defending a client who has been charged with murder. It is alleged
that your client, Mr. Lawton, shot the victim, Mr. Cray. The detective who investigated the scene of the
crime, Mr. Dibny, found a second bullet, from a shot that missed Mr. Cray, that had embedded itself into a
chair. You arise to cross-examine the detective.
You: In what type of chair did you find the bullet?
Dinby: A wooden chair.
You: How massive was this chair?
Dinby: It had a mass of 20 kg.
You: How did the chair respond to being struck with a bullet?
Dinby: It slid across the floor.
You: How far?
Dinby: Three centimeters. The slide marks on the dusty floor are quite distinct.
You: What kind of floor was it?
Dinby: A wood floor.
You: What was the mass of the bullet you retrieved from the chair?
Dinby: Its mass was 10 g.
You: Have you tested the gun you found in Mr. Lawton's possession?
Dinby: I have.
You: What is the muzzle velocity of bullets fired from that gun?
Dinby: The muzzle velocity is 450 m/s.
With only slight hesitation, you turn confidently to the jury and proclaim, "My client's gun did not fire those
shots!"
(a) How are you going to convince the jury and judge?
(b) Choose one part of your solution and perform a sense-making analysis. Clearly state which sensemaking analysis you’ve chosen and why.

It was not fired from the client's gun because the chair slid only 3 centimeters . If it had been fired from the client's gun the chair would slid 25.82 centimeters.

Explanation:

According to the law of conservation of momentum the momentum of the system before collision must be equal to the momentum of the system after the collision.

M1u1= m2u2

Let M1 = mass of the chair = 20kg

m2= mass of the bullet= 10g= 0.001kg

u1= velocity of the chair before collision = zero m/s

u2 = velocity of the bullet before collision = zero m/s

v1= velocity of the chair after collision = ? m/s

v2 = velocity of the bullet after collision = 450 m/s

After collision their velocities change from u1 to v1 and u2 to v2 so

M1v1= m2v2

v1= m2v2/M1

v1= 0.01 *450/ 20= 0.225 m/s

Now according to the law of conservation of energy the energy of the system before collision must be equal to the energy of the system after the collision.

The energy of the chair after the bullet is hit is

KE of the chair + KE of the bullet= 1/2 (M)(v1)²+ 1/2 m(v2)²=

1/2 ( 20) (0.225 )² + 1/2 (0.01) (450)²

= 0.50625 + 1012.5= 1013.00625 Joules

Frictional force = Coefficient of kinetic force of wood on wood ( M+m) g

= 0.2* ( 20.01) 9.8= 39.2196 N

Work done by friction = frictional force * distance

If law of conservation of energy is applied the KE must be equal to the work done

KE = W

W= f*d

KE= F*d

d = KE/f= 1013.00625/ 39.2196= 25.82 cm

The chair did not move 25.82 cm .

It only moved 3 centimeter.

Hence the bullet fired was not from the client's gun.